The distribution wizard header feels out of place with the current XWiki LnF

This is very close to XWIKI-9047 , which discusses this background for updated wikis (that already have a colorTheme).

Here, I think even the default background should be changed to look more similar to something that would be in the default flavor.
The default flavor nowadays uses flat colors, without shadows or anything fancy. This image background feels a bit out of place in comparison to that. I think it'd make XWiki look more professional if we kept somewhat of a consistency on this and used a flat color.